1. Field
The present disclosure relates to a gas detecting device that detects gases having a concentration as low as ppb levels and a method thereof.
2. Related Art
A semiconductor gas sensor is a device that is selectively sensitive to a specific gas, such as, for example, methane gas (CH4), propane gas (C3H8), butane gas (C4H10), or carbon monoxide (CO). The principle and structure of the semiconductor gas sensor are relatively simple, and can be mass-produced easily at low cost.
Such a semiconductor gas sensor is broadly used for detecting a gas leak and incomplete combustion. A gas leak is detected in the order of 1000 to 10000 ppm below a lower explosive limit. Incomplete combustion is detected in the order of 10 ppm to 100 ppm so that a person is not poisoned with CO.
As another use of semiconductor gas sensors, the semiconductor gas sensor is used for analyzing volatile organic compounds (hereinafter referred to simply as VOC). For example, the semiconductor gas sensor is used for analyzing VOC in an indoor environment for the purpose of preventing a sick house syndrome and analyzing VOC contained in the breath that a person exhales for the purpose of managing health conditions.
Examples of VOC include ethanol, methanol, acetone, toluene, xylene, ethyl acetate, formaldehyde, acetaldehyde, chloroform, or paradichlorobenzene contained in paint, printing ink, adhesive, detergent, gasoline, and thinner. In order to realize such analysis, it is necessary to detect VOC in the order of 0.001 to 1 ppm. However, since the conventional semiconductor gas sensor used for detecting a gas leak and incomplete combustion has insufficient sensitivity, it is difficult to detect low-concentration VOC.
Another conventional technique related to a semiconductor gas sensor that detects low-concentration gases is disclosed in patent documents Japanese Patent Application Publication No. 2011-27752 and Japanese Patent Application Publication No. 2011-2358, for example. These patent documents disclose the structure of a semiconductor gas sensor, and also disclose a technique in which a sensor is heated to high temperature and is then cooled to low temperature in order to clean interfering gases adsorbed to a gas sensing film. By doing so, the ability to sense a target gas is enhanced.